Swallowing is a complex process which involves a number of interdependent and coordinated phases. Generally, these include the preparatory, oral, pharyngeal and esophageal phases indicative of the anatomic regions traversed by a swallowed food and/or liquid bolus. During the preparatory phase, a food bolus for example remains in the mouth while it undergoes physical and some chemical changes which make it suitable for transit through the aerodigestive tract. During the oral phase, the bolus is propelled from the mouth into the pharynx by a peristaltic pressure wave generated by sequential squeezing of the tongue against the hard and soft palates. During the pharyngeal phase, the upper esophageal sphincter opens and the bolus is transported into the esophagus by a combination of peristaltic contraction of the pharyngeal constrictors and tongue movement continued from the oral phase. Finally, during the esophageal phase of swallowing, the bolus is transported further into the esophagus and stomach for digestion.
The portion of the swallowing process encompassing the oral to pharyngeal phases is often referred to as oropharyngeal swallowing. Oropharyngeal swallowing begins with closure of the vocal cords, signifying the activation of airway protection, and ends when the vocal cords return to their resting state. Indeed during this time, respiration is reflexively inhibited. Therefore, oropharyngeal swallowing serves two functions, namely transit of the bolus and protection of the airway, in which both functions are highly coordinated.
Due to the complex nature of the swallowing process, pharyngeal and esophageal motor function must operate effectively and in a coordinated manner for a successful swallow to occur. When motor function is compromised, difficulty in swallowing (dysphagia) arises and an ineffective swallow ensues.
Dysphagia is most commonly a consequence of a disease, disorder or condition which impairs coordination, or weakens swallowing biomechanics. For example, dysphagia is often associated with acute events, such as stroke, brain injury, and head and neck cancers, or arises as a result of surgery associated with such cancers. In addition, radiotherapy and chemotherapy associated with cancer treatment tends to weaken the muscles and degrade the nerves associated with the physiology and nervous innervation of the swallow reflex. It is also common for individuals with progressive neuromuscular diseases, such as muscular dystrophy and myasthenia gravis, to experience increasing difficulty in swallowing initiation. Dysphagia is also associated neurological conditions (such as cerebral palsy, Guillain-Barre syndrome, Huntington's disease, multiple sclerosis, Parkinson's disease, and dementia), infectious illnesses, autoimmune illnesses, metabolic illnesses, myopathic illnesses, iatrogenic illnesses, and structural illnesses. Accordingly, dysphagia is generally considered an interdisciplinary phenomenon.
Dysphagia is often accompanied by aspiration due to ineffective airway protection during oropharyngeal swallowing. In effect, food particles, oral secretions and/or stomach contents become misdirected into the larynx and pass into the lungs. Pulmonary aspiration due to swallowing dysfunction (deglutitive aspiration) is the major reason for modification of feeding strategies (e.g. oral to tube feeding, avoidance of liquids etc) which can significantly impact on the quality of life of affected subjects. Furthermore, aspiration can lead to recurrent pneumonia, progressive lung disease, and respiratory disability. Therefore aspiration is a serious condition which can, if undetected, result in severe complications and potentially death. Accordingly, dysphagia and pulmonary aspiration represent significant clinical, social, and economic costs and issues. For example, epidemiological studies estimate a prevalence rate for dysphagia of 16% to 22% among individuals over the age of 50. In addition, dysphagia is extremely common in the paediatric population within a wide range of disorders. This hinders the provision of adequate nutrition, affecting growth and development leading to significant parental anxiety and family disruption. Indeed, in the United States approximately 800,000 individuals per year are affected by dysphagia that is a consequence of neurological disorders, and stroke survivors alone can account for about 100,000 cases of aspiration.
Despite the significantly high prevalence of swallowing disorders and associated complications, the current methods for the assessment of swallowing and for the evaluation of direct aspiration are farfrom optimal. For example, manometry has been used to assess pharyngo-esophageal motor function in a variety of pathologies that cause pharyngeal weakness or impaired upperesophageal sphincter (UES) relaxation. Such disorders lead to ineffective pharyngeal bolus clearance and/or aspiration. The manometric technologies used for this assessment have evolved from single point sensors, to movement-tolerant sleeve sensors and, most recently, high resolution manometry which incorporates multiple closely spaced solid state point sensors. These manometric methods have been utilised to describe the alterations in pressure patterns in relation to well recognized causes of aspiration. These include age-related changes, neurodegenerative disease, post-surgical dysfunctions, and abnormalities of the UES opening due to various factors. The use of manometry for assessment of aspiration risk has been very limited in routine clinical practice. This is because manometric criteria alone have not been shown to accurately assess risk of aspiration and/or post-swallow bolus residue.
Although intraluminal impedance measurement has emerged in recent years as a technique that can be used to detect failed esophageal bolus transport, the application of impedance measurement to examine pharynx motor function has proven extremely challenging. Pharyngeal swallow events occur over a much shorter time span than esophageal peristalsis and several factors cause large variations of the baseline level of impedance, such as variable mucosal contact, residue and secretions. These factors cause impedance signals to be much more noisy in the pharynx than in the esophagus, so that attempts to optimize criteria that identify aberrant bolus flow events and residue have only been partially successful.
Fluoroscopic observation of pharyngo-esophageal bolus transit is the standard tool for evaluation of swallowing function and direct aspiration. The limitations of fluoroscopy are however well known, the most important being prolonged exposure to radiation and the qualitative nature of the test, because it is not possible to derive robust numerical measures. Accordingly it is not appropriate for patient screening. As a result, subjects who are potentially at risk of aspiration are often not referred for fluoroscopy until they have deteriorated clinically and present with weight loss, eating difficulties, recurrent respiratory infections or aspiration pneumonia. Whilst fluoroscopy can identify a point of narrowing of the lumen (such as a stricture, ring or web) that may be impeding normal flow of the bolus, in many patients the test fails to identify any obvious abnormality and these patient are often defined as suffering from non-obstructive dysphagia. Furthermore, even if used for patient screening, there is clear evidence that fluoroscopy is poorly predictive of progression to aspiration pneumonia, and due to limits on investigation time, a normal fluoroscopy cannot entirely guarantee the absence of feed aspiration.
Indeed, at present there is no method that is sensitive for identification of subjects at high risk for deglutitive aspiration at a time when aspiration-associated complications might be prevented by intervention. Even observed clinical signs and symptoms (such as wet voice, wet breathing, and cough) have only a 33-67% sensitivity to predict aspiration of liquids on fluoroscopy. Furthermore, fluoroscopy-based parameters, such as pharyngeal residue, are relatively poor markers of aspiration.
On the basis of the aforementioned inadequacies of existing techniques, there is a substantial interest in developing new and effective methods which enable an assessment of swallowing function in individuals, so as to identify those individuals with ineffective swallowing (for example due to a functional abnormality causing an obstruction), and who are therefore at risk of aspiration.
Reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any country.